Intraindividual Variability, Cognition, and Aging
Researchers examining cognitive functioning in adulthood have typically focused their attention on average age-related effects. Methodologically, this emphasis has translated into comparisons of mean level performance across different age groups using cross-sectional designs or examination of average changes in performance over time using longitudinal designs. Research on average age-related differences and changes in cognition has been useful, but it reflects certain assumptions about the nature of human development. Specifically, this emphasis is rooted in the assumption that either the behaviors of interest are stable over time or that the trajectory of change that does occur is similar for all persons. This assumption with respect to level of performance represents one instantiation of a more general stability perspective that has dominated developmental research (Gergen, 1977; Nesselroade & Featherman, 1997). By contrast, variability in performance, particularly variability within persons, has received less attention. However, as noted by Nesselroade and Boker (1994), the concepts of stability and variability are logically dependent on one another — defining one demands consideration of the other.